getcontext

GETCONTEXT(3)              Linux Programmer's Manual             GETCONTEXT(3)

NAME
       getcontext, setcontext - get or set the user context

SYNOPSIS
       #include <ucontext.h>

       int getcontext(ucontext_t *ucp);
       int setcontext(const ucontext_t *ucp);

DESCRIPTION
       In  a  System  V-like environment, one has the two types mcontext_t and
       ucontext_t defined in <ucontext.h> and the four functions getcontext(),
       setcontext(),  makecontext(3), and swapcontext(3) that allow user-level
       context switching between multiple threads of control within a process.

       The mcontext_t type is machine-dependent and  opaque.   The  ucontext_t
       type is a structure that has at least the following fields:

           typedef struct ucontext_t {
               struct ucontext_t *uc_link;
               sigset_t          uc_sigmask;
               stack_t           uc_stack;
               mcontext_t        uc_mcontext;
               ...
           } ucontext_t;

       with  sigset_t  and stack_t defined in <signal.h>.  Here uc_link points
       to the context that will be resumed when the current context terminates
       (in case the current context was created using makecontext(3)), uc_sig-
       mask is the set of  signals  blocked  in  this  context  (see  sigproc-
       mask(2)),  uc_stack  is  the  stack  used  by this context (see sigalt-
       stack(2)), and uc_mcontext is the  machine-specific  representation  of
       the  saved  context,  that includes the calling thread's machine regis-
       ters.

       The function getcontext() initializes the structure pointed at  by  ucp
       to the currently active context.

       The  function setcontext() restores the user context pointed at by ucp.
       A successful call does not return.  The context should  have  been  ob-
       tained by a call of getcontext(), or makecontext(3), or passed as third
       argument to a signal handler.

       If the context was obtained by a call of getcontext(),  program  execu-
       tion continues as if this call just returned.

       If the context was obtained by a call of makecontext(3), program execu-
       tion continues by a call to the function func specified as  the  second
       argument  of  that  call to makecontext(3).  When the function func re-
       turns, we continue with the uc_link member of the structure ucp  speci-
       fied  as  the first argument of that call to makecontext(3).  When this
       member is NULL, the thread exits.

       If the context was obtained by a call to a  signal  handler,  then  old
       standard  text  says that "program execution continues with the program
       instruction following the instruction interrupted by the signal".  How-
       ever,  this  sentence  was removed in SUSv2, and the present verdict is
       "the result is unspecified".

RETURN VALUE
       When successful, getcontext() returns 0 and setcontext() does  not  re-
       turn.  On error, both return -1 and set errno appropriately.

ERRORS
       None defined.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +---------------------------+---------------+------------------+
       |Interface                  | Attribute     | Value            |
       +---------------------------+---------------+------------------+
       |getcontext(), setcontext() | Thread safety | MT-Safe race:ucp |
       +---------------------------+---------------+------------------+
CONFORMING TO
       SUSv2, POSIX.1-2001.  POSIX.1-2008 removes the specification of getcon-
       text(),  citing  portability issues, and recommending that applications
       be rewritten to use POSIX threads instead.

NOTES
       The earliest incarnation of this mechanism was the setjmp(3)/longjmp(3)
       mechanism.   Since that does not define the handling of the signal con-
       text, the next stage  was  the  sigsetjmp(3)/siglongjmp(3)  pair.   The
       present mechanism gives much more control.  On the other hand, there is
       no easy way to detect whether a return from getcontext()  is  from  the
       first  call,  or via a setcontext() call.  The user has to invent their
       own bookkeeping device, and a register variable won't do  since  regis-
       ters are restored.

       When  a signal occurs, the current user context is saved and a new con-
       text is created by the kernel for the signal handler.  Do not leave the
       handler  using  longjmp(3): it is undefined what would happen with con-
       texts.  Use siglongjmp(3) or setcontext() instead.

SEE ALSO
       sigaction(2),  sigaltstack(2),  sigprocmask(2),  longjmp(3),   makecon-
       text(3), sigsetjmp(3)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                     GETCONTEXT(3)